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National Centers for Environmental Prediction (NCEP)
Hydrometeorlogical Prediction Center (HPC) Forecast Operations Branch
Winter Weather Desk
Dan Petersen
Dan.Petersen@noaa.gov
301-763-8201
Goals of Presentation
• Review HPC winter weather desk forecasts and verification
• Review verification of snow to liquid ratio forecasts
• Review Short Range Ensemble Forecast Temperature Biases and Upcoming Changes
HPC Winter Weather Desk• Issues heavy snow and icing forecasts and collaborates these
forecasts with local Weather Forecast offices September 15- May 15
– Internal 24-hour Snow and Ice accumulation probability graphics for days one through three.
– Public and technical versions of low tracks graphic depicting synoptic scale surface low tracks associated with heavy snow or ice.
– Heavy Snow and Ice Discussion
• Forecasts and discussions posted at http://www.hpc.ncep.noaa.gov/wwd/winter_wx.shtml
HPC Products and ServicesWinter Weather Desk
Probabilistic Snow Forecasts are issued for 4, 8 and 12 inches, and icing for 0.25 inches
HPC Winter Weather Desk Day 3 probabilistic snow forecast verification
2006-8
24
25
26
27
28
29
30
4" 8"12"
HPC forecaster improvement over Brier skill
score for all NCEP model and
ensemble members
(2007-2008 top, 2006-2007 bottom)
2007-8
2006-7
0
5
10
15
4" 8"12"
73
74
75
76
77
78
Day 1Day 2Day 3
2007-8
36
38
40
42
44
46
48
Day 1Day 2Day 3
HPC Winter Weather Desk Days 1-3 probabilistic 0.25” ice forecast
verification 2006-8
HPC forecaster improvement over Brier skill
score for all NCEP model and
ensemble members
(2007-2008 top, 2006-2007 bottom)
2006-7
The displayed model and ensemble low positions show the
uncertainty of the low positions- the black line represents the
HPC preferred track
HPC Products and Services-Winter Weather Desk Low Tracks Graphic
HPC Products and ServicesLow track verification 2007-8
0
50
100
150
200
250
12 24 36 48 60 72
HPC NAM SREFMN GEFSMN GFS CMC UKMET ECMWF
ECMWF had lowest rms
errors 48-72 hours, followed
by UKMET
HPC Winter Weather Forecast Process• Snow accumulations obtained by multiplying experimental
snow to liquid ratios times quantitative precipitation forecast (QPF)
• Experimental ratios blend the UWM (Roebber et al) neural net technique, climatology, and fixed snow to liquid ratios
HPC Winter Weather Desk Forecast ProcessClimatology of Snow to Liquid Ratios (courtesy Marty
Baxter)
http://www.eas.slu.edu/CIPS/SLR/slrmap.htm
Snow to Liquid Ratio References
The Morphology of Snow Crystals and Crystal Density
Libbrecht, http://www.its.caltech.edu/~atomic/snowcrystals/
Snow to liquid ratio studiesRoebber et al (2003)
Snow ratios determined by temperature, relative humidity, solar radiation (month), and compaction (via wind speed and weight of liquid
forcing snow to compress)
10-member ensemble of artificial neural networks assigns snow ratio in one of three classes: heavy (ratio < 9:1), average (9:1 to 15:1), and light
(ratio > 15:1).
Real time Web page interface for calculating SLR
http://sanders.math.uwm.edu/cgi-bin-snowratio/sr_map_main.pl
Role of Vertical Velocity in determining snow ratio?
• The maximum growth rate is expected to occur near the level of maximum upward air motion within the cloud, where the greatest water vapor delivery occurs (Auer and White 1982).
Cobb 21st Conference on Weather Analysis and
Forecasting A simple physically based snowfall algorithm
http://ams.confex.com/ams/WAFNWP34BC/techprogram/paper_94815.htm
Cobb (Caribou) Method • SLR is
computed for each cloud layer and then weighted towards the layer with the greatest vertical velocity (UVV)
Verification of the Snow to Liquid Ratio Forecast Tools
Verification of the Snow to Liquid Ratio Forecast Tools
Winter 2006-7 and 2007-8 Snow to Liquid Ratio Verification Summary for the SLR methods run using GFS model output and verified against cooperative observer and CoCoRahs reports
0
1
2
3
4
5
RMS Error Bias
Day 1 Caribou
Day 1Climatology
Day 1 UWM
Community Collaborative Rain, Hail & Snow Network www.cocorahs.org
Verification of the Snow to Liquid Ratio Forecast Tools
Verification of the Snow to Liquid Ratio Forecast Tools
Winter 2006-7 and 2007-8 Snow to Liquid Ratio Verification Summary for the SLR methods run using NAM model output
-1
0
1
2
3
4
5
RMS Error Bias
Day 1 Caribou
Day 1Climatology
Day 1 UWM
Short Range Ensemble Forecast (SREF) Temperature Forecast Issue and Upcoming
Changes
• Non Hydrostatic Mesoscale Model (NMM)
• Advanced Research Weather Research Forecast (ARW)
SREF NMM Member 2m Temperature Bias Dec. 1, 2007-
Feb. 29, 2008
-5
-4.5
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0 6 12 18 24 30 36 42 48 54 60 66 72 78 84
NMMCTL NMMMN1 NMMP1
SREFMean 2m temperature bias Dec. 1, 2007-Feb. 29, 2008
SREFMean (Blue) often
the coldest bias vs. the NAM (black) gfs (red), bias corrected srefmean (green)
GFS
NAM
SREFMEAN
SREF 2m Temperature upgrade impact:
• Increase in WRF to ten members
- Non Hydrostatic Mesoscale Model (NMM) increase to five members (if temperature bias uncorrected, would have greater impact on mean)
- Advanced Research Weather Research Forecast (ARW) increase to five members
Mean forecast error of 2m Temp (C) of parallel SREF (left), current operational
SREF (right) June 16-26 2008
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0 6 12 18 24 30 36 42 48 54 60 66 72 78 84
NMMCTL NMMMN1 NMMP1
-1.8-1.6
-1.4-1.2
-1-0.8
-0.6-0.4
-0.20
0.2
0 6 12 18 24 30 36 42 48 54 60 66 72 78 84
NMMCTL NMMMN1 NMMP1
The results from the parallel experiment show a slight positive bias the first 48 hours, trending to a slight negative bias 54-84 hours
(green line is zero bias)
SREF Mean 2m temperature analysis03z Sun 10/26/08
Area of concern-central to northern
Rockies and high Plains
2m SREF Mean temperature 84 hr forecast at 03z 10/26/08
84 hour operational
forecast shows minima of 0C (red) where no areas analyzed
less than 5C
2m SREF Mean parallel temperature forecast
84 hour parallel SREF forecast
shows minima of 5C (still a cold
bias but improved over prior SREF by 3-4 degrees C)
12z Tue. Oct 28 2008 SREF operational (green), parallel (red) 27 hour
forecast sea level pressures on left screen, 12z HPC surface analysis (right)
Sea level pressures 4-6 mb too high in much of NY-PA-NJ-New England in both operational, parallel sref
12z Tue. Oct. 28 2008 500 mb 27 hour forecast sref operational (green), parallel (red) left screen, initial global data assimilation
analysis (right)
500 mb low verified near PA-NJ border with both SREF versions showing low near PA-MD border
Upcoming SREF changes December 2008
• Upgrade both NMM and ARW to version 2.2 and increase their model horizontal resolution from 40/45km to 32km
• Upgrade RSM model version and increase resolution from 45km to 32km
• Increase WRF membership from 6 to 10 (5 NMM and 5 ARW) and reduce Eta membership from 10 to 6
• Enhance physics diversity among RSM members: replace Zhou cloud scheme with Ferrier scheme for 3 SAS members
• Increase output frequency from 3hrly to hourly for the 1st 39hr to meet the demand from aviation and convection forecasting
• Add aviation and wind-variance fields to ensemble products• Breakout “big” SREF bufr output into individual stations
Short Range Ensemble Forecast (SREF) derived probabilities for snowfall rates,
amounts, and precipitation type• http://www.spc.noaa.gov/exper/sref/
• Select most recent model run, then winter weather option on left
Probability of snow as
precipitation type for 8 am EDT Friday
April 11 2008
SREF Derived Winter Weather Impact Graphics-derived probabilities for snow fall
rates, visibilities, and event duration• http://www.hpc.ncep.noaa.gov/wwd/impactgraphics/
SREF probability of one inch per
hour snowfall 00z Friday
April 11 2008
SREF Temperature Bias Dec. 1 2007-Feb 29 2008
HPC Products and ServicesLow track verification 2007-8
0
50
100
150
200
250
HPC NAM SREFMN GEFSMN CMC UKMET ECMWF
12 hr
24 hr
36 hr
48 hr
60 hr
72 hr
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